Check all of your connections@ your gen for corrosion.Clean tighten and apply dielectric grease lightly to these areas as needed.Use caution when tightening,secure the lower nut,and snug the upper down until the connection is tight. Be careful not to turn the studs as you can damage these connections internally.Is your gen grounded? small flathead screw to the inside close to the doghouse it may also have an 8mm or smaller head as well.Examine the connections @ your voltage regulator for loose or dirty ends. Once again clean and tighten apply dielectric grease as needed.Follow this step by step from the gen to the light in your speedo cluster and examine grounds that you encounter along the way.Another area of interest is the ground strap from tranny to chassis examine it.Points to ponder,you may be way ahead of me in this process.

Since you are running an alternator with an internal regulator, run a new 8 gauge wire from the battery lug on the starter to the Alternator 1st. Then run another 8 gauge wire to your head light switch from the alternator. You could also run a new 18 gauge wire for the idiot light at the same time as well. This would eliminate all the 40 year old wiring form the charging system.

This was the same problem I had on mine and it took care of the problem.